The present invention relates to a mechanism for extending and contracting an antenna, and more particularly a mechanism for manually or automatically extending and contracting an antenna, for example a telescope type antenna.
As is well known, a telescope type antenna is constituted by a plurality of tubes having different diameters which are interconnected like a telescope. The antenna is extended or contracted by using a wire which does not extend and contract. The wire utilized for this purpose has a circular cross-sectional configuration. It is advantageous that the wire is straight when the antenna is extended. A synthetic resin, for example a polyacetal resin, is suitable to form an operating wire having a circular cross-section.
In recent years, a gear is used for extending and contracting the operating wire. In this case, the gear is situated at a wire passing port provided for a casing containing a drum or at a position just below an antenna base cylinder mounting portion. The gear is driven by an electric motor and a rack is provided for engaging the gear. Special constructions for driving the gear are disclosed in Japanese Utility Model Laid Open Patent Specification Nos. 49,942/1974, 155,033/1971 and 36,613/1979. In these references, a strip member or the like is substituted for a wire driven by the gear, and perforations are provided for the strip member for engaging the gear. Furthermore, Japanese Laid Open Patent Specification No. 29,047/1978 discloses a plastic belt clamped between rollers to be extended and contracted by driving the rollers.
According to Japanese Laid Open Patent Specification No. 90,951/1979, a gear is formed on periphery of a large diameter pulley contained in a casing, and driven by an electric motor, and a wire push member is provided in a range of 90.degree. in the peripheral direction of said pulley starting from a position immediately beneath an antenna base cylinder, that is an antenna receiving cylinder. Furthermore, a rack is formed on one side of said wire so as to mesh the rack with the gear provided for the pulley. This reference also shows a construction wherein the rear surface of the wire push member is made flat so as to efficiently guide the wire push member.
Further, Japanese Utility Model Laid Open Patent Specification No. 31,044/1981 discloses a flat rectangular wire which is substituted for a conventional wire having a circular cross-section for the purpose of neatly accommodating an antenna operating wire in a casing. Since this wire is intended to substitute for a conventional circular wire, its cross-sectional configuration is approximately square.
As is well known, the extending and contracting antenna is frequently used in motor cars. Such car mounted antenna is extended and contracted by manual operation or automatic operation utilizing an electric motor. The manual operation has been used for many years. With the motor operation, the antenna can be extended and contracted by operating a switch installed in a car. Especially, it is convenient when the antenna is expanded and contracted in an interlocked relation with the ON.multidot.OFF operation of a car radio switch and when it rains, the driver is not required to get out of the car for manipulating the antenna. Actually, however, the driver may often get out of the car without contracting the antenna. In such a case, the driver must enter again into the car for operating a switch so as to contract the antenna. For manually or automatically extending and contracting the antenna various proposals have been made. For example, where a multithread worm is used, it can be driven by manual operation and an electric motor. According to another proposal, a planet wheel mechanism is controlled by an electromagnet for extending and contracting the antenna.
As above described, according to the prior art construction, an antenna operating member is made of a noncontractive linear synthetic resin wound about a rotary drum. Thus, the antenna is extended and contracted by the rotation of the drum through the antenna operating member. Accordingly, not only the drum diameter is increased but also the resistance to paying out and taking up of the antenna operating element having a relatively large resistance is increased. For this reason, the size of the antenna extending and contracting mechanism is determined by the diameter of the drum, meaning an increase in the capacity of the motor. The wire is payed out while being strongly urged against the inner surface of the drum or the surface of the winding groove (which is arcuate for receiving the wire having a circular cross-section). But the wire tends to slide along the inner surface of the drum or of the winding drum, while being inclined thereto. Thus, the pay out efficiency caused by a push force decreases substantially. Slipping of the wire while being urged against the inner surface of the drum or the surface of the winding groove means pushing out the wire from the rotatably mounted drum toward a stationary base member which causes the periphery of the drum to move away from the base member. According to operating conditions (especially, when the drum is not clutched off after it has reached the paying out limit or when the telescope type antenna is frozen in a cold season) the wire would be pushed in between the drum and the base member like a wedge, thus disenabling the rotation of the drum in either direction. Furthermore, the wire would be twisted helically in an opening through the antenna base cylinder, thus breaking the wire.
In a construction utilizing a gear and if a rack and the operating member is payed out and taken up by the gear driven by a motor, the disadvantages described above can be obviated. However, in this construction, the mechanism for driving the gear with the motor becomes bulky. In a press molded rack or strip shaped rack formed with openings for engaging the gear, the longitudinal strength of the rack is small, thus failing to achieve a smooth extension and contraction of the antenna. Especially, a strip shaped rack has a tendency to bend thus failing to obtain optimum extension and contraction. In a construction where a plastic belt is clamped between opposing rollers, not only the construction becomes complicated but also the above described disadvantage of the strip shaped member can not be obviated.
In the construction disclosed in Japanese Laid Open Patent Specification No. 90,951/1979, since a relatively long wire is payed out or taken up by the rotation of a pulley while being strongly urged against a stationary guide member, the friction between the wire and the guide member increases, thus disenabling smooth extension and contraction. Especially when the rear surface of the wire opposing the rack is made flat, these members contact with each other with a relatively large area thus increasing the friction therebetween. Especially in a strip formed with a rack or holes for engaging a gear, not only the number of the manufacturing steps increases but also sufficiently large longitudinal strength can not be obtained. For this reason, the cross-sectional area of the strip increases.
The operating wire disclosed in Japanese Laid Open Utility Model Specification No. 31,044/1981 has substantially square cross-section. Although it can be neatly taken up in a drum, in the taken up state, the wire tends to bend in the circumferential direction of the drum so that the taken up state of the wire is not always stable. Accordingly, the push up force of the antenna and the extending and contracting characteristics are similar to a circular wire whereby smooth take up and miniaturization of the mechanism are difficult.
Where a multithread worm is used for manually or automatically extending and contracting the antenna, the advantage of a low speed drive is decreased. Further, it is necessary to provide another speed reduction mechanism between the worm and the take up drum thereby increasing the capacity of the motor. When the motor operated system is operated manually, the motor operates as a generator so that it is necessary to provide a protective device for the source of the motor. A mechanism utilizing plat gears is required to be incorporated with a permanent magnet and a control switch for ON.multidot.OFF controlling the permanent magnet at a suitable timing. In each case, for preventing excessive load applied to the operating wire at the limits of extension and contraction of the antenna, it is necessary to provide a clutch mechanism for releasing the same when an overload condition occurs. As above explained, the construction of the prior art manual or motor operated antenna extending and contracting mechanism is complicated and its cost of manufacturing is high.